GEOLOGY OF LAKE PLEASANT QUADRANGLE 25 



For 2 miles up Ninemile creek from its mouth, there are many 

 outcrops of pink to red granite but all the rock is massive and 

 homogeneous, though very gneissoid, except at one locality, namely, 

 one-quarter of a mile above the gabbro stock or dike where am- 

 phibolite inclusions cause the rock to appear much like that just 

 described as occurring on Hamilton Lake stream. 



The vicinity of West hill, southeast of Wells, should be men- 

 tioned as another good place for the study of the pink granite 

 with inclusions because of numerous exposures in open fields. 

 This granite is not always homogeneous, but shows variations 

 occasionally to granitic syenite or even to normal syenite. There 

 are many inclusions, three being of sufficient size for separate map- 

 ping as Grenville. The largest inclusion is of hornblende gneiss 

 much like that so often associated with Grenville limestone, while 

 the two smaller inclusions are of Grenville white acidic gneiss. 

 Many ledges show streaks, bands, or thin lenses of gray or white 

 gneisses exhibiting all stages of assimilation by the granite. 



Granite and syenite porphyries 



These rocks are practically identical in composition and appear- 

 ance with the granite porphyries recently described by the writer^ 

 as occurring in the Broadalbin and North Creek quadrangles. On 

 the accompanying map six areas of these rocks are shown, these 

 being confined to the northern and eastern portions of the quad- 

 rangle and comprising some 12 or 13 square miles of its area. 



In the table on page 26 the mineralogical compositions of several 

 carefully selected examples are given. A comparison with the 

 granite shows essentially the same mineral variations and constants 

 except that at times the quartz content is so low that the rocks are 

 more properly syenite porphyries. Such less quartzose porphyries 

 are of minor extent. 



The porphyritic texture is generally pronounced with phenocrysts 

 of potash feldspar up to an inch long standing out clearly, though 

 usually these phenocrysts are rather well granulated. The degree 

 of foliation varies widely from moderate to such an extreme that 

 the quartz and feldspar crystals are well flattened out. 



As seen on the weathered surface, the color is nearly always 

 pinkish to reddish gray, and generally this color seems to pervade 

 the whole rock, though in the bed of Devorse creek a ledge of very 

 fresh granite shows a greenish gray color. 



IN. Y. State Mus. Bui. 153, p. 17-20. N. Y. State Mus. Bui. 170, 

 p. 21, 22. 



